• S Venugopal Rao

      Articles written in Pramana – Journal of Physics

    • Picosecond optical nonlinearities in symmetrical and unsymmetrical phthalocyanines studied using the Z-scan technique

      S Venugopal Rao P T Anusha L Giribabu Surya P Tewari

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      We present our experimental results on the picosecond nonlinear optical (NLO) studies of symmetrical and unsymmetrical phthalocyanines, examined using the Z-scan technique. Both the open-aperture and closed-aperture Z-scan curves for three samples were recorded and the nonlinear coefficients were extracted from the theoretical fits. The nonlinear absorption/refraction contribution from the solvent was also identified. The observed open aperture behaviour for these molecules is understood in terms of the absorption coefficients of these molecules near 800 nm and the peak intensities used. It is established that these phthalocyanines exhibit large optical nonlinearities and, hence, are suitable for optical limiting applications.

    • Femtosecond laser-fabricated microstructures in bulk poly(methylmethacrylate) and poly(dimethylsiloxane) at 800 nm towards lab-on-a-chip applications

      K L N Deepak S Venugopal Rao D Narayana Rao

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      Laser direct writing technique is employed to fabricate microstructures, including gratings (buried and surface) and two-dimensional photonic crystal-like structures, in bulk poly(methylmethacrylate) (PMMA) and poly(dimethylsiloxane) (PDMS) using $\sim 100$ femtosecond (fs) pulses. The variation of structure size with different writing conditions (focussing, speed and energy) was investigated in detail. Diffraction efficiencies of the gratings were calculated and the changes in diffraction efficiency (DE) as a function of period, energy and scanning speed were evaluated. Highest diffraction efficiencies of 34% and 10%, for the first order, were obtained in PMMA and PDMS respectively. Heat treatment of these gratings demonstrated small improvement in the diffraction efficiency. Several applications resulting from these structures are discussed. Fs modification in PMMA and PDMS demonstrated emission when excited at a wavelength of 514 nm. We attempted to prepare buried waveguides in PMMA with higher refractive index at the core. We have successfully fabricated branched and curved structures in PMMA and PDMS finding impending applications in microfluidics.

    • Ultrashort laser pulse–matter interaction: Implications for high energy materials

      S Venugopal Rao

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      The interaction of ultrashort [nanosecond (ns)/picosecond (ps)/femtosecond (fs)] pulses with materials is an exhaustive area of research with underlying, and often extremely rich, physics along with a plethora of applications evolving from it. High-energy materials (HEMs) are chemical compounds or mixture of compounds which, under suitable initiation, undergoes a very rapid exothermic and self-propagating decomposition. Herein, we describe the interaction of laser pulses with materials and its implications for studies on HEMs in four parts: (a) ns and fs laserinduced breakdown spectroscopic (LIBS) studies of HEMs towards understanding the molecular dynamics and discrimination, (b) ps/fs pulses interaction with metallic solids towards the production of nanoparticles, nanostructures and their utility in identifying explosive molecules using surface-enhanced Raman scattering studies, (c) interaction of laser pulses with the bulk and surface of glasses and polymers producing micro- and nanostructures for microfluidic/lab-on-a-chip applications, and (d) ultrafast spectroscopic studies for comprehending the excited state dynamics towards elucidation of vibrational dynamics in HEMs. Several applications resulting from these interactions will be discussed in detail.

    • Synthesis of Cu2O, CuCl, and Cu2OCl2 nanoparticles by ultrafast laser ablation of copper in liquid media

      Syed Hamad G Krishna Podagatlapalli Surya P Tewari S Venugopal Rao

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      Copper complex nanoparticles were fabricated from bulk copper using picosecond laser ablation in water and chloroform. We found that composition of the nanoparticles was CuCl and Cu2OCl2 in chloroform at three different input fluences; Cu2O in water which was confirmed from the data of EDAX, UV-Visible absorption spectra, and selected area electron diffraction pattern. We have also performed nonlinear optical studies of colloidal nanoparticles using Z-scan technique at 800 nm and ∼2 ps laser pulses. Cu2O NPs exhibited two-photon absorption at lower peak intensities while three-photon absorption was observed at higher peak intensities. Other samples exhibited two-photon absorption at all peak intensities.

    • Interaction of ultrashort pulses with molecules and solids: Physics and applications

      S Venugopal Rao

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      The interaction of ultrashort laser pulses with molecules and solids is an extremely complex area of science research encompassing the fields of physics, chemistry, and materials science. The physics of interaction has been fairly understood over the last couple of decades and, consequently, several applications have been envisaged from these interactions in the fields of photonics, lithography, biomedicine, sensing, telecommunications etc. In the present article we describe three different components of interaction of ultrashort pulses with matter: (1) with liquid molecules/thin films wherein we present the results from our studies of optical nonlinearities predominantly using picosecond and femtosecond pulses, (2) with molecules/solids wherein plasma generated from the surface was studied for applications in understanding the molecular dynamics and towards identifying high-energy molecules and (3) within the bulk and on the surface of solids (e.g. glasses, bulk polymers and metals) resulting in micro- and nanostructures. Different applications resulting from such interactions in photonics and microfluidics are presented and discussed.

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